Both nitric acid vapors and NO2 are highly toxic. The US Department of Labor Occupational Safety and Health Administration (OSHA) has set stringent guidelines aimed at protecting workers performing operations in an environment potentially contaminated with both fuming nitric acid/nitric acid vapors and NO2. The Permissible Exposure Limit (PEL) for NO2 has been established at 10 mg/m3 (approximately 5 ppm). The Permissible Exposure Limit (PEL) for fuming nitric acid/nitric acid vapors has been established at 5.2 mg/m3 (approximately 2 ppm).
Impregnated, activated carbon is known to strongly adsorb a wide variety of toxic vapors from ambient air streams. Impregnated, activated carbon does not, however, effectively filter nitric acid vapors and NO2 from ambient air streams. In the case of NO2, there is no mechanism associated with impregnated activated carbon capable of removing NO2. NO2 will, however, react with the surface of carbon to yield NO, which rapidly elutes from the carbon bed. NO, like NO2 is toxic. The PEL for NO has been established at 30 mg/m3 (approximately 25 ppm). Being acidic, one would expect nitric acid vapors to be removed by impregnated activated carbon, such as those employed in military gas mask filters. Nitric acid vapors; however, are not removed by these filters because nitric acid vapors react with the surface of activated carbon to yield NO, which is toxic (PEL for NO has been established at 30 mg/m3, approximately 25 ppm).
Filtration media capable of removing NO2 from streams of air is limited. Gooray et al. (U.S. Pat. No. 4,680,040) describe the use of alkali metal silicates, specifically sodium silicate, lithium silicate and potassium silicate, to adsorb NO2. The reaction is believed to involve the cation of the alkali metal silicate combining with NO2 to form an alkali metal nitrate. Although data demonstrates this material to be effective in filtering low NO2 concentrations (up to about 10 mg/m3), the material does not appear to be able to filter high concentrations of NO2 (1,000 mg/m3).
Onitsuka et al. (U.S. Pat. No. 5,158,582) describe the use of copper salts (CuCl, CuCl2, CuCl2.[NH4Cl]2) impregnated into zeolites to filter NOx (mixtures of NO and NO2). The copper impregnated zeolites are used to remove low levels of NOx (about 10 mg/m3) present in highway tunnels (from automotive exhaust). Up to 140 minutes of service life is reported for 10 mg/m3 challenge and 1 mg/m3 breakthrough. The patent notes that humidity in the air greatly reduces the performance of the material, and the process described within the patent consists of an air drier up-stream of the NOx adsorber.
Ichiki et al. (U.S. Pat. No. 5,840,649) describe the use of a Mn doped TiO2 impregnated with either Ru or Cu oxide, plus Ag, Cu or additional Mn. The material is reported to be useful in treating low levels (about 10 mg/m3) of NOx from moist air streams.
Amines are known to interact with NO2. A possible reaction between amines and NO2 has been reported by Diaf et al. (J. Appd. Polymer Sci. 53 (1994) 857) and Gaden (AIChE Symposium Series No. 309 (1995) 49–60). This reaction is presented below:RNH2+NO2→RN—NO+H2O
However, under conditions of high relative humidity, the amine-NO2 reaction is minimized because pores of many adsorbents, such as carbon, become saturated with water. For toxic vapors such as NO2 that are not highly soluble in water, the presence of water in the pores of the adsorbent greatly decreases the performance of the filtration media.
Therefore, none of these previous solutions provide an effective, low cost means of removing both nitric acid vapors and NO2 from ambient stream of air under varying temperatures and humidities.